Just finished installing a shower in an existing bathroom that
previously had only toilet & sink.
1-piece shower shell placed in corner against 2 existing sheet rock
walls. Added floor-to-ceiling stud wall on 3rd side of shower.
I've calked and screwed the shell to the 3 walls. Now I'm not sure
what the best way is to trim above the shell and along the front edges
where it is attached to the 2 side walls.

Cedar trim.

Paint the wall first, then rabbet the back side of the trim so it sits flat
over the shower flange, nail it up (careful not to nail through the shower
flange), and cover with a couple of coats of spar urethane.

Normally, you wouldn't want to use wood near a shower, but this should be
outside the majority of the wet areas. In any case, the cedar would hold up
better than the drywall in the area.

Installing trim would also let you remove and replace the shower stall
easily in the future if you needed to (let's hope that's not any time
soon).

Of course, you could paint the trim to match your room, or use a router to
create a decorative profile. Your choice.

Anthony